The present invention relates to pumps and control systems therefore. More particularly, the present invention relates to a double-acting, duplex pump having a power side controlled by two, two position spool valves.
Pumps are used in a wide variety of industries to deliver fluids. For instance, the delivery of fluids at high pressure may be accomplished with an intensifier pump. Intensifier pumps are commonly used in a variety of industries where the delivery of fluids at a high pressure is desired. For example, intensifier pumps may be used in conjunction with subterranean operations to deliver cement slurries, stimulation fluids, drilling fluids, or other fluids at the desired pressure. Also, in offshore operations, an intensifier pump may be used to remove the hydrostatic head from seawater that is applied to the downhole fluids. In these instances, the intensifier pump may be located subsurface.
Generally, intensifier pumps are reciprocating, fluid-driven apparatuses that comprise one or more large pistons connected to one or more small pistons. Intensifiers are powered by a hydraulic fluid, such as water. In a common intensifier pump, a double-acting, low-pressure, power chamber contains a central piston slidably disposed therein having a high-pressure piston extending from each face. The high pressure pistons extend oppositely from the central piston out of the power chamber into a high-pressure, pumping cylinder. In operation, hydraulic fluid is directed into the low-pressure, power chamber in such a manner to cause the central piston to reciprocate back and forth. The central piston, in turn, drives the high-pressure pistons, which alternately pump an intensified fluid at high pressures to a desired location. As will be understood by those skilled in the art, intensification of the fluid occurs, because the area of the central piston is larger than the area of the high-pressure pistons.
When the central piston reaches one end of its stroke, there may be a short delay in the flow of high-pressure fluid. This delay is due to a necessary precompression of the fluid in the high-pressure, pumping cylinder to the operating pressure. Due to this short delay, there may be a pressure dip in the output from the high-pressure, pumping cylinder. This pressure dip is undesirable, inter alia, because it may damage the power system, the pump, the fluid flow lines, and/or the well. To counteract these pressure dips, a double-acting, duplex intensifier pump may be used, wherein two central pistons may be operated in parallel so that one central piston is on its power stroke while the other central piston is changing direction and/or is in a precompression stroke. The operation of the double-acting, duplex intensifier pump must be controlled with precision because the pressure dip may be severe if both central pistons reach the end of their power strokes simultaneously. Therefore, the timing of the power stroke and precompression stroke of the two central pistons should be controlled to provide a substantially constant discharge pressure from the high-pressure, pumping cylinders. One or more control valves may be provided to control the supply of hydraulic fluid to and venting of hydraulic fluid from the low-pressure, power chambers that contain the two central pistons. Conventional control systems may be hydraulically activated. However, timing issues may occur with the hydraulically activated system, thereby disrupting the compression and precompression cycles of the pump. Problems also may be encountered with control systems for pumps other than intensifier pumps.